Many optical fiber communication systems comprise semiconductor laser radiation sources. The operation of many of such lasers can be adversely affected if reflected radiation is permitted to impinge on the laser. This is particularly true for such high performance lasers as single mode single frequency lasers used in some current or proposed high bit rate fiber transmission systems. Exemplarily, reflections typically also have to be controlled in 2-way fiber communication links and in multi-channel analog fiber communication systems. Control of reflections thus is an important consideration. See, for instance, R. Rao et al., Electronics Letters, Vol. 22(14) pp. 731-732 (1986), incorporated herein by reference.
A known method of preventing significant amounts of reflected power to reach a laser comprises polishing of a fiber end such that the resulting endface is tilted with respect to the fiber axis (ibid), as illustrated schematically in FIG. 1. The Rao reference shows the relationship between the angle of tilt of the fiber endface and the reflected power. For instance, for a tilt angle of 3.degree. the reflected power is about -35 db, and for 5.degree. it is about -45 db.
FIG. 1 shows the relevant portion of a fiber joint 10 between a first fiber 11 and a second fiber 12. Radiation 13 from a laser source propagates towards the joint, with typically a large fraction of the radiant energy entering the second fiber and propagating away from the joint, as indicated by means of arrow 14. The endfaces of the fibers are polished such that a given endface has a tilt angle .phi. (preferably .phi..gtoreq.5.degree.) relative to the fiber axis. As is the case wherever radiation encounters a discontinuity in refractive index, a part (15) of the radiation is reflected at the first fiber endface, and a part 16 of the radiation is transmitted through the endface and impinges on the second fiber endface, with a portion 17 being reflected, and a portion 14 being guided in the second fiber. For appropriately chosen .phi., radiation 15 cannot be guided by fiber 11 and exists from the fiber, as indicated. Furthermore, for an appropriately by chosen tilt angle, radiation 17 does not enter the core of fiber 11.
Fiber-to-fiber joints are not the only fiber joints that can comprise a prior art tilted fiber endface, as is schematically illustrated by FIG. 2. The Figure shows a joint 20 which comprises a semiconductor laser 21, with radiation 23 being emitted from active region 22 and being focussed onto the tilted endface of fiber 26 by spherical lens 24. A portion 27 of incident radiation 25 is reflected at the endface, and portion 28 is propagating in guided fashion away from joint 20 towards some utilization means that is not shown. If the tilt angle of the endface is appropriately chosen, reflected radiation 27 will not be focussed onto 22 and thus does not interfere with system operation.
The above embodiments of fiber joints that comprise at least one tilted endface such as to substantially reduce reflections are exemplary only. Other embodiments are known, and still others may be developed in the future. As those skilled in the art will recognize, not all reflected and/or refracted rays are shown in FIGS. 1 and 2.
A prior art tilted endface is produced by a technique that comprises introducing the fiber into a fixture that has an appropriately tilted polishing surface such that the fiber end extends beyond the polishing surface, possibly breaking the fiber, and removing the portion of the fiber that extends beyond the polishing surface by a, typically conventional, polishing procedure.
Although effective for preventing reflections, the prior art technique of producing a tilted endface by polishing has disadvantages. Among these are substantial cost, the possibility of sub-surface damage, fiber breakage and contamination. In view of these drawbacks it would be desirable to have available a method for producing a tilted endface that is not subject to shortcomings of the prior art technique. This application discloses such a method.